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If you're talking disposalable freon tanks consider this............they have a one way check valve to stop anything from entering the tank. Plus the valve orifice size may be too small for good air flow. You might have to remove the valves and use another fitting. How will you drain the tanks of moisture? I have heard of people using water heater tanks for air but I would think by the time they get them they would be crap. I personally think you may be wasteing your time to use freon tanks and try to coat the inside. The reason
for the one way valves was to stop people from doing this. Now if you can find some old aluminum freon tanks, that would be great. I doubt you will. They probably have all be taken to the recycle spots already.

When you have a air flow thru a void, its bound to pick up some material, not sure of how much, but some. Plus I have to drain my tanks every so often for moisture, that may be a concern for you using these freon tanks.

My plasma cutter require two air filters on the input of the cut, while I keep those on the cutter. The air line to the plasma cutter also has filters. It keeps the moisture down and the cutter doesn't complain about foreign matter.

Air Tank

Their are codes to go by when designing and building pressure vessels, even small ones for air. You can buy some Sch. 10 Pipe and some caps. Then you have the cylinder shape. Boxed , (Square) is a very bad Idea . Having the metal rolled somewhere might be an option also, and buying some dished heads for the ends. Keep it safe with a relief device.

I saw a set up once with a gang of 3 old Argon/CO2 welding tanks set up for a air compressor.

These tanks also were short a few holes for drains, outlets, inlets, relief valves, etc. The guy mounted them upside down in a rack with the outlets at the lowest point.

Then, there was a reducing bushing, a nipple screwed in each one, and a few 'T's. The various 3rd points on the 'T's became outlets, inlets, cross overs, pressure sensor for the pump, and 'T's in the horizontal connecting pipes between the tanks had a relief valve, and finally a single drain outlet on the lowest point of one end. It was well thought out, and the manifold going across the bottom did everything it was supposed to do.

"Gone are the days of wooden ships, and Iron men.
I doubt we'll see either of their likes again".